Over the past decades, farmers have been getting bigger harvests from the same size plot of land. That story’s particularly true in the American Midwest, the world’s corn basket. The U.S. now grows more than 36 percent of the world’s corn.

One key is that breeders developed corn plants that can grow very close together. Newer varieties can better withstand stress and grow roots deeper to access water. So stalks are planted more densely and yields per acre are up.

“What we did see for corn is that progress has been much greater for what we consider good weather conditions, so cooler, wetter conditions.” Study leader David Lobell of Stanford University, on Science’s podcast. “And when you get to the hottest conditions we see actually very little progress…the simple observation was that the sensitivity to those hot conditions seems to be growing over time.”

So in the hotter, drier climate predicted for the coming decades, the varieties of corn that currently produce bumper crops could be in trouble. “The impacts of drought are actually growing, at least in the case of maize.”